In MR imaging, it is often desired to obtain information about the relative contribution of different chemical species, such as water and fat, to the overall signal, either to suppress the contribution of some of them or to separately or jointly analyze the contribution of all of them. These contributions can be calculated if information from two or more corresponding echoes, acquired at different echo times, is combined. This may be considered as chemical shift encoding, in which an additional dimension, the chemical shift dimension, is defined and encoded by acquiring two or more echoes at different echo times. In particular when applied to the separation of the contributions of water and fat to the overall signal, these types of experiments are often referred to as Dixon-type of measurements. In general such a separation is possible because there is a known precessional frequency difference of hydrogen in fat and water. In its simplest form, Dixon imaging or Dixon water/fat imaging generates water and fat images by either addition or subtraction of the ‘in phase’ and ‘out of phase’ datasets. However, this so-called 2-point Dixon technique fails, when B0 field inhomogeneities become larger. This is the case in many clinical applications at high B0 fields where global shimming cannot completely compensate for local field variations. 3-point or 4-point Dixon techniques were developed to correct for these field inhomogeneities. These techniques provide, in addition to a water image and a fat image, a map of B0 field inhomogeneities, the so-called B0 map.
In the known Dixon-type water/fat imaging methods, multiple MR images are acquired at different echo times, wherein each echo signal is conventionally collected in a separate sequence repetition. This increases the minimum scan times by a factor corresponding to the number of different echo time values. In more recent implementations, all echo signals are acquired in a single sequence repetition, that is after a single excitation, by using appropriate multi gradient echo imaging sequences, thereby significantly reducing the required scan times. So-called ‘unipolar’ imaging sequences may be applied to acquire all echo signals using the same magnetic field gradient polarity. This mostly ensures phase consistency among the echo signals. Alternatively, so-called ‘bipolar’ imaging sequences may be applied in which the echo signals are collected during both positive and negative magnetic field gradient polarities. This has several advantages. On the one hand, the so-called ‘fly-back’ magnetic field gradients between the echo signal acquisitions can be dispensed with which improves the signal-to-noise (SNR) efficiency. On the other hand, the minimum required repetition time of the imaging sequence and, thus, scan time can be significantly shortened. Moreover, the minimum echo spacing (echo time increment) can be reduced, whereby the spectral bandwidth in which water/fat can be unambiguously determined is increased. This results in a more robust water/fat separation. However, the bipolar imaging sequences must account for phase errors that result from eddy currents and other system non-idealities. In unipolar acquisitions, the phase errors effectively add a constant phase on all the echo signals which can easily be compensated for, since the relative phases between the echo signals remain unchanged. In bipolar acquisitions, the phase errors effectively add a different constant phase on all the echoes acquired with positive magnetic field gradient polarity and negative magnetic field gradient polarity, thereby disrupting the phase consistency between the individual echo signals which is critical for water/fat separation. In addition, even in unipolar acquisitions, the first, or the first few, echo signals are often affected by further phase errors.
Various strategies have been proposed to minimize the effect of phase errors in bipolar acquisitions. Among them is the approach of performing a correction of the acquired echo signals based on additional calibration measurements (see Yu et al. S, Journal of Magnetic Resonance Imaging 31, 1264-1271, 2010). However, performing an additional calibration measurement to estimate and correct the phase errors requires extra scan time, especially since the phase errors are spatially varying. Further, the paper ‘Fat quantification using multi-echo sequences with bilopar gradients: investigation of accuracy and noise performance’ in MRM71(2014)219-229 by P. Peterson and S. Månsson mentions that off-resonance effects, T*2 relaxation and eddy current effects are taken into account to correct for phase errors in fat quantification.